Although most places generally honor the major diving certifications for all organizations, it is best to check with the individual tour operators to see for sure. For the western Caribbean I would suggest you go with a private tour and not the ship since it will be cheaper. Although the standard is PADI or NAUI, most places accept anything that shows you've been trained. But looking around I don't see ACUC listed in the universal referral program that most places use. Perhaps you could do a check dive with a nearby PADI or NAUI center and get certified with them? That would make it a bit neater since all the diver tours accept PADI and NAUI.